Former President Trump has promised that, if reelected, he will kick out millions of immigrants living in the U.S. illegally.

Trump and his surrogates have offered sparse details for how he would carry out the “largest deportation operation in American history,” but have cemented the goal as a top priority. What is known: The strategy would rely on military troops, friendly state and local law enforcement, and wartime powers.

“No one’s off the table,” Tom Homan, Trump’s former head of Immigration and Customs Enforcement, said in July. “If you’re in the country illegally, you better be looking over your shoulder.”

Republican vice presidential nominee JD Vance said the administration would start by deporting immigrants who have committed crimes.

At a campaign rally earlier this month in Aurora, Colo., Trump said he would invoke the Alien Enemies Act of 1798 “to target and dismantle every migrant criminal network operating on American soil.”

The ex-president went on to say that he would send “elite squads” of federal law enforcement officers to “hunt down, arrest and deport” every migrant gang member. Those who attempt to return to the U.S. would be served with 10-year prison sentences without parole, he said, adding that any migrant who kills a U.S. citizen or law enforcement officer would face the death penalty.

How many people would Trump go after?

It’s unclear.

In May, Trump told Time magazine he would target 15 million to 20 million people who he said are living illegally in the U.S. The nonpartisan Pew Research Center estimates the actual number to be about 11 million as of 2022. More than 2 million people have entered the country illegally since then.

“Let’s start with 1 million,” Vance told ABC News in August.

During his entire presidency, from January 2017 to January 2021, Trump deported about 1.5 million immigrants, according to a Migration Policy Institute analysis of federal figures — far fewer than the 2 million to 3 million he speculated about deporting in a 2016 interview as president-elect. The Biden administration is on pace to match Trump’s deportation numbers.

What powers would Trump invoke to justify deportations?

The Alien Enemies Act of 1798 allows the president to arrest, imprison or deport immigrants from a country considered an enemy of the U.S. during wartime. Congress passed the law as part of the Alien and Sedition Acts — four laws that tightened restrictions on foreign-born Americans and limited criticism of the government, when the country was on the brink of war with France.

The law has been used three times in American history: during the War of 1812 and World War I and after the attack on Pearl Harbor during World War II.

During WWI, federal authorities placed 6,300 “enemy aliens” — many from Germany — into internment camps.

By the end of WWII, more than 31,000 people from Japan, Germany and Italy, as well as some Jewish refugees from Nazi Germany, had been interned at camps and military facilities — in addition to the more than 100,000 Japanese Americans who were forcibly relocated to the same camps and detained under different legal grounds, said Gabriel “Jack” Chin, a UC Davis professor who studies criminal and immigration law.

Chin said he isn’t convinced that Trump would make the Alien Enemies Act the cornerstone of his immigration policy because the U.S. is not in a declared war with another nation.

“It would have to rest on an argument that random immigration — that is to say immigration based on individual decisions of individual people — is the equivalent of an invasion from a nation-state,” he said. “And that would have to be based on an idea that foreigners as a group are a nation.”

Trump has also said he would deploy National Guard troops under the orders of sympathetic governors.

“If I thought things were getting out of control, I would have no problem using the military,” he told Time.

Federal law limits the involvement of military troops in civilian law enforcement.

In 2018, Trump sent 5,800 active-duty troops to the southwestern border amid the arrival of a caravan of thousands of migrants from Central America. Initially the troops performed support work such as laying razor wire as a deterrent to crossing, but later the White House expanded their authority to allow them to use force and provide crowd control to protect border agents.

Last year, President Biden sent 1,500 Army and Marine Corps troops to fill critical “capability gaps” at the border as the administration lifted the Title 42 border expulsions policy that Trump had invoked to turn away asylum seekers and other would-be immigrants as the COVID-19 pandemic raged.

Trump has promised to go further during a second term by recalling thousands of troops from overseas to be stationed at the U.S.-Mexico border. He has also explored using troops to assist with deportations and confronting civil unrest.

Is it legal?

Using the Alien Enemies Act, Trump could conduct rapid deportations without the typically required legal processes. He could also circumvent federal law to use military troops in a broader law enforcement capacity to carry out arrests and removals.

But speeding up the deportation process could come with catastrophic consequences, Chin said. Scores of U.S. citizens are already mistakenly deported.

“If the point of this was a roundup, U.S. citizens would be rounded up,” he said.

Katherine Yon Ebright, an attorney at the Brennan Center for Justice, argued in an analysis of the law that courts would likely avoid opining on the presence or absence of an invasion, or whether the perpetrator of the alleged invasion is a foreign nation or government.

“The courts’ hesitance to weigh in on these questions heightens the risk that Trump will invoke the Alien Enemies Act despite its clear inapplicability,” she wrote. But she added that “courts may strike down an invocation of the Alien Enemies Act under modern due process and equal protection law, justiciable grounds for checking abusive presidential action.”

Tom Jawetz, deputy general counsel at the Department of Homeland Security from 2021 to 2022, said courts tend to give deference to the president for executive determinations. But he said this one could be difficult to uphold.

“There could be opportunities for legal attack,” he said. “It sounds like they would be stretching it beyond its capacity, beyond what the text [of the law] would allow.”

Is it feasible?

Deporting millions of people would be expensive and logistically complex.

Former President Obama, who in 2013 oversaw the most deportations in a year when his administration kicked out 438,000 immigrants, relied on local police turning people over to federal immigration agents. Trump has said he would similarly rely on state and local law enforcement. But many state and local governments, including California, have since limited their cooperation with immigration agents.

Immigration courts are already overwhelmed, and more deportation cases would add to the backlog of 3.7 million cases. Lengthy delays in immigration court proceedings mean immigrants often wait years before their case is completed.

Among the rights afforded to immigrants is a 2001 Supreme Court ruling that prohibits them from being indefinitely detained if their country won’t accept them back. Countries including Venezuela and China have previously refused to cooperate with U.S. authorities on deportations.

How much would it cost?

It would cost at least $315 billion to deport the roughly 13 million people in the country illegally, according to an analysis by the American Immigration Council, a group that advocates for policies that welcome migrants. The deportation effort would require building hundreds of new detention facilities, as well as hiring hundreds of thousands of new immigration agents, judges and other staff.

Immigration and Customs Enforcement’s budget last year was about $9 billion. Significantly increasing its funding would require the backing of Congress — an uphill battle given current political divisions.

Jawetz said Trump could redirect funds from the Federal Emergency Management Agency and the Department of Defense, like he did for construction of the border wall, and could also reassign personnel from other agencies to perform immigration enforcement tasks.

An analysis by CBS News found that it cost an estimated average of $19,599 to deport one person over the last five fiscal years after apprehension, detention, immigration court processes and transport out of the U.S. were taken into account. The average cost of repatriation only increases as more migrants arrive from distant countries such as Cameroon and China.

How are people preparing?

Mass deportation could rip apart deeply rooted families that include citizens and noncitizens, worsen labor shortages and lead to economic upset. Discussion of mass deportation alone would also sow fear in immigrant communities, as happened during Trump’s first term.

Jawetz said advocates for migrants are beginning to consider potential legal action. During Trump’s presidency, informal Signal and WhatsApp networks emerged across the country in which advocates and community members communicated real-time responses to policy changes they were seeing on the ground.

“We would hope and expect to see much of the same this time around” if Trump wins, the former Homeland Security counsel said. “If you think about it, just the level of anxiety people [would be] living under on a day-to-day basis over a period of years is pretty extraordinary.”